High-frequency heating systems and applicators



Feb. 26, 1957 H. R. WARREN 2,783,347

HIGH-FREQUENCY HEATING SYSTEMS AND APPLICATORS Filed March 26, 1954 s Sheets-Sheet 1 Feb. 26, 1957 H. R. WARREN HIGH-FREQUENCY HEATING SYSTEMS AND APPLICATORS Filed March 26, 1954 3 Sheets-Sheet 2 Feb. 26, 1957 H. R. WARREN 3 Sheets-Sheet 3 Mutual Inductance (microhenries) 4 3 3 2 2 :5 muozo uupzoom 2:3:

Electrode Voltage Columbus, Ind., assignor to National Company, Chicago, Ill., a corporation Henry R. Warren, Cylinder Gas of Delaware Application March 26, 1954, Serial N 0. 419,074 18 Claims. or. 219 10.ss

This invention relates to high-frequency heating and particularly to dielectric heating systems and resonant applicators which are particularly suited for heating of large area loads, such as foam rubber mattresses, wallboard panels, groups of sand cores or plastic preforms, and the like.

This application, a continuation-impart of my application Serial No. 138,628, filed January 14, 1950, and now abandoned in favor of my continuation-impart application Serial No. 419,633, filed March 26, 1954, has claims dirooted to subject matter divided from my parent application Serial No. 138,628. My aforesaid applications contain a more detailed discussion, included herein by this reference, of the structural and operating characteristics of resonant applicators of the kind to which the present invention is particularly directed.

In general, such applicators comprise relatively large electrode structures electrically interconnected through conductive structure which at least in part has substantial inductance cooperative with capacity-means including the capacitance between said electrode structures to form a resonant circuit device, and a power transfer coupling loop disposed in position to be traversed by a highfrequency magnetic field encircling a part of said interconnecting conductive structure. More particularly, in the preferred form of such applicators, the interconnecting structure includes low resistance conductive walls of a. shielding enclosure which completes the resonant circuit of said device and serves to confine said magnetic field and also the electric field produced between said electrode structures, at least one of the electrode structures being electrically interconnected with wall structure of the enclosure through 'a leg or fin element which reentrantly projects into the enclosure and inwhich a substantial part of the inductance of the resonant applicator is concentrated, and the said coupling loop being arranged to be traversed by the-magnetic field encircling an inwardly projecting leg or finielement. Such loop. may serve to provide for excitation of' the applicator by an oscillator having the loop in its anode circuit.

In accordance with the present invention, the mutual inductance of the applicator and coupling loop is adjusted by variation. of the effective area of the loop. which may be movable in whole, or. in part, or which may be provided with a shorting bar, a vane, or other conductive structure movable relative to a stationary part of the loop.

Further in accordance with the present invention, piping providing a supply of coolant for the anode of an oscillator tube may be. attached to and made a conforming part of the stationary portion of the coupling loop in avoidance of rotary or flexible fluid connections and of radiofrequency chokes. Usually, and preferably, such piping is-attached to the loopstructure in such locations as to serve as alcorona guard andto cool contact areas between movable and stationary portions or components of the loop.

The invention further resides in features of combination,

2,783,347 Patented Feb. 25, 1957 construction and arrangement hereinafter described and claimed.

For a more detailed understanding of the invention and for illustration of various embodiments thereof, reference is made to the accompanying drawings, in which:

Fig. l is an elevational view, in cross section, of a press applicator and an associated oscillator circuit schematicaliy shown;

Fig. 2 is a perspective view, partly broken away, of another applicator and an associated oscillator circuit schematically shown;

Fig. 3 is a perspective view showing details of construction of a coupling loop arrangement;

Fig. 4 is an elevational view, in cross section, of another applicator and an associated oscillator circuit schematically shown;

Fig. 5 is a perspective view showing details of construction of another coupling loop arrangement;

' Fig. 6 is an elevational view, in cross section, of another applicator and an associated oscillator circuit schematically shown;

Fig. 7 is an elevational view, in cross section, of an applicator embodying another coupling loop arrangement;

Fig. 8 is a perspective view disclosing in detail the coupling loop arrangement of Fig. 7; and

Figs. 9 and 10 are explanatory figures referred to in discussion of the adjustment or operation of the applicators of preceding figures.

The reentrant resonant press applicator IOA-of Fig. 1 is particularly suited for production of laminated plywood sheets of substantial area, for production of large sheets by edge-bonding of wooden strips, and for like purposes requiring large heating electrodes and dissipation of relatively large amount of radio-frequency power in the dielectric load. The press frame is in the form of a metal tunnel 11A elongated in direction normal to the plane of Fig. l.

The lower, wide and elongated face of a beam attached along its upper end to the inner face of the upper wall of the tunnel and forming part of the press frame serves as the stationary electrode 16A of the applicator. The movable electrode 15A of corresponding length and width is supported by or upon the upper ends of the vertical plungers or rams 17A spaced in parallel rows along the tunnel housing.

For scarf-joining of wood, gluing of the laminations of plywood, drying of wallboard and like uses, the press platens or electrodes are fiat plates. For molding or shaping of dielectric material, the press platens are suitably embossed or shaped to obtain the desired configuration of the object or objects being heated between them. The platens may be in the form of rolls, smooth-faced. or embossed, for. pressure-rolling of dielectric sheeting or strip which is concurrently heated between the roll-platens.

The actuating cylinders 18A of the rams 17A which are provided for applying pressure in direction normal to the opposed faces of the platen-electrodes 15A, 16A may be disposed in the press-foundations 12A. A series of cylinders A18 for applying lateral pressure to work 19A between the electrodes 15A, 16A extend externally of the housing 11A along one of the side walls. The stroke of the side rams may be small and the tunnel adapted for a wide range of load widths by provision of manually adjustable pressure screws 56A along'the opposite side wall.

The resonant frequency or wavelength of the applicator 10A is predominantly determined by the inductance and capacity of the press-applicator itself. Specifically, the capacitance of the power-receiving or work circuit is internally of thehousing 11A and chiefly that between the opposed surfaces of' the electrodes 15A, 16A; and theinductance of that circuit is internally of the tunnel grease? and is predominantly that of the conductive press structure within the tunnel between its upper and lower walls. More particularly, the reentrantly extending web or central fin 13A of the stationary frame member is a significant portion of the total tunnel inductance and substantially all of the remainder consists of the inductance of portions of plungers 17A Within the tunnel.

The resonant press-applicator A may be, and preferably is, the frequency-determining or tank circuit of a self-excited oscillator which forms the high-frequency power supply means having a power-delivery circuit which includes the power-transfer coupling loop 51A. In Fig. 1, the oscillator system 24A is of the so-called T. N. T. type (tuned anode, non-tuned grid) in which the grid-anode capacity of the oscillator tube provides the feedback coupling required for generation of oscillations.

Specifically, one terminal of the grid inductance 26 is connected to the grid of tube 25 and the other terminal thereof is connected to the cathode of the tube by high frequency by-pass condensers 27, 30. The grid-leak resistor 28 is traversed by the rectified grid-current of the tube to derive the direct-current bias of the grid from the generated oscillations. The grid resistor 28 is shunted by the by-pass condensers 27, 30 in series. In the seriesfeed arrangement shown, the anode of the tube 25 is connected through inductance 51A to one terminal (8+) of a suitable high-voltage power source whose other terminal (B) is connected to the cathode of the tube 25. The exterior of the press-applicator is at ground potential for both the high radio-frequency and the high direct-current voltages involved in dielectric heating.

'The resonant press-applicator 10A is inductively coupled to the anode circuit of the oscillator tube 25 by the loop 51A which is disposed within the tunnel and threaded by the high-frequency magnetic field encircling the web or fin inductor 13A within the applicator housing 11A. This loop is adjustable about a vertical axis to vary its area normal to the lines of the aforesaid highfrequency magnetic field so to permit smooth variation of the mutual inductance between the anode circuit and applicator 10A. The effective area of the loop 51A may, however, be varied in other ways including those herein specifically mentioned. For reasons later herein more fully discussed, the range of adjustment of mutual inductance preferably should be entirely in the supraoptlmum range.

Conductor 35 from the anode of the oscillator tube 25 extends through an insulator 105A in a wall of the tunnel to one terminal of loop 51A and the other termmal of the loop is grounded to a tunnel wall. Conductor 35 should be very short and of low impedance at the oscillator frequency.

To maintain proper grid-excitation for safe operation of tube 25 despite variations in electrical load occurring during heating of dielectrics or during adjustment of the electrode, the inductance of grid coil 26 may be automatically adjusted by motor 52 in response to variations 1n magn1tude of the rectified grid-current or self-derived gnd bias of oscillator tube 25. The block 53 is generically representative of the arrangement for that purpose disclosed and claimed in my Patent 2,517,948.

In the press-applicator of Fig. 1, the framework should be strong and heavy enough to resist the stresses and strains imposed, and since no insulators are included in the plunger-platen system, this construction is particularly suited for very heavy presses. When the tunnel applicator is to be used for work requiring application of little or no pressure, its construction, as in types sub sequently described, may be considerably simplified and lightened.

In the dielectric-heating applicator 108 (Fig. 2), the upper heating electrode 163 and the walls of the tunnel 11B may be of relatively thin sheet metal such as aluminum. These elements of the applicator are of large sur- V 4 face area and thus, although the total circulating current in the tunnel may be very high, as over a thousand amperes, the current density in the wall structure is low. However, the current density is higher in the fin inductor 13B connecting the hot electrode 16B to the tunnel wall structure and consequently this fin structure, which may be a single wide sheet or a row of straps, should be of high conductance. In the particular fin construction shown in Fig. 2, it comprises a single elongated sheet of copper or copper alloy attached at its upper end to a supporting bar or frame member 5613 in turn attached, as by welding, to the upper wall of the tunnel 11B and extending parallel to the side walls thereof. The lower end of fin 13B is attached to a similar conductive member 578 extending lengthwise of the upper heating electrode 16B. The edges of the fin 13B are so shaped or so spaced from the end walls of the tunnel as to leave an unobstructed path around the fin for its high-frequency magnetic field. Such unobstructed path should exist in all tunnel applicators when the ends of the tunnel are completely or partially closed.

The flexibility of the fin structure 1313 permits the upper electrode to be raised or lowered to accommodate work loads of different physical or electrical characteristics or when spaced from the load to adjust the magnitude of the potential gradient from top to bottom of the Work. Suitable devices, exemplified by the rods 17B, in whole or in part of insulation, are provided for moving the upper electrode. The bottom of the tunnel may serve as the lower electrode 15B. Alternatively, the lower electrode may be an auxiliary conductive member, movable or stationary, conductively connected or otherwise coupled to the tunnel wall structure.

The ends of tunnel 11B may be left open, at least part way up from the bottom, for insertion, removal or passage through the tunnel applicator of the objects or material 19B to be heated, as by means of a conveyor such as indicated at 63B. For batch operation, one or both ends of the tunnel 11B may be provided with metal doors or movable panels.

In accordance with my aforesaid parent applications, the resonant applicator 10B (Fig. 2) may be, and preferably is, coupled to an oscillator tube 25 to serve as the tank or frequency-determining circuit of a self-excited oscillator system 24B. Specifically, the coupling between the oscillator tube 25 and the resonant tunnel applicator 10B is efiected by a loop 51B disposed in the high-frequency magnetic field of the fin 13B. The degree of coupling, as later discussed, is preferably supraoptirnum throughout the range of adjustment of the loop.

In the oscillator system 24B of Fig. 2, the grid-excitation of the oscillator tube is derived from the potential between the heating electrodes 15B, 16B by a capacity voltage-divider 59, 62, such as claimed in my said parent applications. The reactance of capacitor 59 is usually substantially greater than the reactance of the effective input capacity 62 of the oscillator tube 25.

With loop 51B adjusted to provide the desired radiofrequency heating voltage on electrode 16B and with capacitor 59 preadjusted or preset for proper grid-excitation, the capacity 62 has an effective value which, as fully explained in my aforesaid parent applications inherently varies with the applicator loading so that the ratio of the two reactances of the capacitor voltage-divider varies auto matically with load and in proper sense to stabilize the grid current.

An applicator generally similar in construction to Fig. 2, having height, length and width approximately of 3, '12 and 8 feet, with an electrode 16B having length and width of 10 and 5 feet respectively, has been in operation at frequencies of from about 12 to 16 megacycles for dielectric heating of pump wallboard requiring dissipation in the load of radio-frequency power of the order of kilowatts and radio-frequency potentials between the heating electrodes of the order of 25,000 volts. The presn'ected to the tunnel nearer inductor, the potential of the hot electrode is approxisure applied was light but sulficient to prevent warping of the wallboard during heating.

Oscillator tubes for operating at such high powers are usually of the liquid-cooled anode type giving rise to the problem (in series-feed oscillator circuits such as 24A, 24B of Figs. 1 and 2) of supplying water or other coolant to the tube jacket 109 (Fig. 3). The arrangement shown in Fig. 3 provides for a continuous supply of coolant without danger of high-voltage flash-over and without need for recourse to choke coils or to use of insulated piping and distilled water. The pipes 107, 10%, Within the tunnel, are attached to and conform with the shape of a wide metal strap 110 and serve both to prevent corona and overheating of the strap. The anode end sections of pipes 107, 103 are electrically insulated from the tunnel by their passage through a sheet of insulation 105. The opposite end sections of the pipes, which are metallic throughout, are grounded to a tunnel wall.

The coupling loop A51 is a composite one including a stationary loop section formed by the pipes 107, 108 and strap 110 and a shunt swinging link 51L for varying the coupling between the tunnel and the anode circuit of the oscillator tube. The position of shunt link 51L shown in Fig. 3 closely corresponds with the maximum coupling setting. The adjustment of link 51L may be effected through a cord, drum and pulley arrangement (111, 112, 113). The shaft of drum 112 extends externally of the tunnel for drive by motor 116 or other actuating means. It is to be noted this arrangement avoids need for leakproof rotary of fiexiblejoints in the adjustable coupling loop.

In Fig. 4, the resonator 10C is similar to those previously described but the associated oscillator circuit 24C is of different type. Either the inductance 26 or the condenser 72, or both, may be varied to control the magnitude of the oscillator grid current or voltage for proper excitation.

Supraoptimum coupling of the anode coil 51C to the resonant applicator 10C preferably should be used, for reasons hereinafter discussed, to minimize variation of the voltage of the heating electrode 16C with changes in effective power-factor of the applicator. The coupling may be varied by adjustment of the shorting bar 670 or by other arrangements herein described.

The fin inductor 13C is of construction permitting raising and lowering of the electrode to accommodate work 19C of different thickness or to vary the air gap above the work. Specifically, the tin 130 may include an extensible section of flexible conductor shaped to form accordion-like folds or fabricated from a multiplicity of strips edge-joined to form such folds. tion is fully disclosed and claimed in my aforesaid parent applications.

For supplying coolant to the tube jacket when the adjustable element of the coupling loop is a shorting bar, the arrangement of Fig. 3 may be simplified as shown in Fig. 5. As before, the supply and discharge pipes 107, 103, Within the tunnel, are attached to and conform with the shape of a Wide'metal strap 110 to form the coupling loop and serve both to prevent corona and overheating of the strap. The anode end sections of pipes 107, 108 which extend to the tube jacket 109 (Fig. 3) are insulated from the tunnel by their passage through a sheet 105 of insulation. The opposite end sections of the pipes, which are metallic throughout, are grounded to the tunnel. The shorting bar 67 is slidable along the loop and may beheld in adjusted position by any suitable means exemplified by clamping devices E66 of known type cooperating with slots in the loop strap Lil-ll.

When, as in Fig. 4, the anode end of the loop enters 'the tunnel housing the grounded end of the fin inductor and the opposite end of the loop leaves or is conthe electrode end of the fin mateiy 180 out of phase with respect to the anode p0- Such a constructenti'al and in usable for grid-excitation through a cac'a'city-coupling as in Fig. 2. However, the heating electrode voltage may, for high power tunnels, be above the breakdown voltage of commercially available condensers having the required high current rating.

As described and claimed in my parent application Serial No. 419,633, the oscillator circuits of Figs. 1 and 4, as in Fig. 6, the coupling coil may be reversed so that it is in part formed by the top tunnel wall. This reversed loop arrangement provides closer coupling between the anode circuit and applicator and also increases the frequency of the anode circuit. The inductive coupling may be varied by adjustment of the strap 74D connecting the fin 13D to the adjacent side of loop 51D and/or by adjusting the shorting bar'67D.

For supplying coolant to the tube jacket in an arrangement such as shown in Fig. 6, the arrangement of Fig. 3 may be used in simplified form with strap and pipes 107, 108forming only two sides of the coupling loop.

The applicator 10E shown in Fig. 7 is generally similar to those previously described and is used as the tank circuit of the oscillator system 24B of Fig. 2. The loop 51E for coupling the applicator to the anode circuit of the oscillator tube comprises an upper stationary portion 51F and a lower portion 51M which is pivoted for rotation about a horizontal axis through an arc of approximately from the position of maximum loop area. The variation in coupling between the oscillator anode circuit and the fin 13E changes the potential applied to the load 19E.

With the other arrangements described, the distance from the lower side of the coupling loop to the hot electrode must be sufiicient to avoid flash-over with the hot" electrode in its uppermost position. When the electrode'has a substantial range of movement, for example, a foot or more, it is therefore necessary substantially to restrict the vertical dimension of the loop and therefore the maximum loop area to meet the flash-over requirement between the loop and the electrode. Consequently, with the electrode at the lowermost position, the magnetic flux density within the loop area was substantially reduced with correspondingly lower available maximum coupling or mutual inductance. With the loop arrangement shown in Fig. 7, as the electrode is lowered, the eiiect of reduced flux density upon maximum available coupling may be offset by swinging the adjustable portion 51M of the loop downwardly to increase the maximum loop area which may be used, Without danger of flashover to the electrode.

A preferred arrangement for supplying coolant to the oscillator tube when the coupling loop is of this type is shown in Fig. 8. The stationary portion 51F of the loop 515 comprises the metallic sections of pipes 107, 108 attached to and conforming with the wide copper strap 110. These pipe sections serve as corona shields and the coolant therein is efiective to prevent overheating of this portion of the loop. To avoid need for leak-proof rotary or flexible connections, as would be necessary if pipes E07, 108 were incorporated in the rotatable series-loop element 51M, the pipe sections in the vertical, right-hand side of stationary loop member 51F are offset from the axis of rotation of rotatable loop member 51M and the lower horizontal sections of the pipes extend therefrom directly to the tunnel wall. The inner bearing 117 of the rotatable loop member 51M is cooled by shaping at least one of the pipes (108) partially to surround the bearing and contact fingers 118. The outer bearing, at the tunnel wall, may be similarly cooled. With this construction, the lower horizontal sections of piping from the lower end of the fixed loop member 51F to the tunnel wall should be of glass or other good insulation and the coolant should be of low electrical conductivity, such as distilled water.

In all of the resonant applicators herein described, all, or practically all, of the capacitance is concentrated between the heating electrodes and all','or practically all, of the inductance is concentrated in the fin structures electrically; interconnecting the heating electrodes with the wall structure; In each of them, the walls of the applicator housing serve as a low resistance, low reactance path for thecirculating currents of the fin inductance and the capacitance of the heating electrodes. As the resistances of the inductive and capacitive elements of the applicator are low, as is also the resistance of the wall structure, the Q is very high despite the high ratio of capacitance to inductance.

Because of their high frequency, the circulating currents are practically confined to the inner surfaces of the applicator housing. Consequently all external surfaces of the housing are at ground potential and serve as a radio-frequency shield for the high-frequency fields within the housing. Therefore, the radiation losses are low which minimizes radio interference and further contributes to high Q of the applicator. Because of its very high Q, this type of applicator is uniquely suited for dielectric load materials having very low-power-factor such as foam rubber, extruded rubber hose and the like while at the same time being suitable for material of higher powerfactor, such as wood.

In configuration, the applicators of Figs. 1 to 7 may be considered, in a geometric sense, as forming a substantially rectangular enclosure having a transversely elongated fin extending inwardly and downwardly from the top wall structure to an elongated upper heating electrode. Such fin may be of any of the types disclosed and claimed in my copending application Serial No. 419,071, which is a divisional continuation of my said parent'applications. With such construction, the heating electrodes though of large area to accommodate a work load of correspondingly large rectangular shape, neither enforce use of frequencies which are too low for efficient heating nor require the use of stubs to minimize excessive voltage gradients along the electrodes.

With tunnel applicators such as herein disclosed, it has been proved possible to obtain efficient uniform heating of very low power-factor loads requiring radio-frequency energy at high-power levels, even in excess of 100 kilowatts. Satisfactory heating has been accomplished, with commercially available tubes, in the range of about to 50 megacycles, for which frequencies large heating electrodes may be used without stubbing. In contrast with the resonant load circuits heretofore used for dielectric heating at these frequencies, the tunnel applicator has an exceptionally high unloaded Q (Qs of over 5,000 are obtainable), alfording unusually high electrode voltage without attendant excessive circuit losses. Therefore, even with dielectric loads having a power-factor much less than 1%, the percentage of high-frequency power delivered to the tunnel and utilized in useful heating of the dielectric load is of the order of 90%.

By way of specific example, a tunnel applicator having an unloaded Q" of 2,750, with a 5-foot by 10-foot electrode of 370 micromicrofarads capacity, and operating at a frequency of 14 megacycles with a peak electrode potential of 32,000 volts, delivered 148 kilowatts in heating a load having a power-factor of 0.9% with a power loss of only 6 kilowatts in the tunnel circuit. An oscillator using a conventional load circuit having an unloaded Q of 200, and delivering the same power (148 kilowatts) to an identical load would be forced to supply 83 kilowatts of wasted power to the load circuit.

"*As more fully described and claimed in my aforesaid parent application, Serial No. 419,633, a significant advantage of the tunnel applicator is that it is particularly suited for supraoptimum coupling to he anode circuit of a self-excited oscillator which as now explained is helpful in minimizing change in heating electrode voltage with change of power-factor of the applicator and in minimizing any tendency of the oscillator-frequency to jump upon change in load with consequent failure of transfer of power to the work-load.

The curves of Fig. 9 illustrate the variation of heatingelectrode voltage of a particular tunnel applicator for various degrees of coupling, or mutual inductance, for three examples of loading. In each curve, the maximum heating-electrode voltage occurs at a point 0 termed the point of optimum coupling at which the effective resistance Rb, reflected into the anode circuit of the oscillator tube from the tunnel applicator, is equal to the effective anode resistance Rp. The corresponding optimum mutual inductance is given by the equation:

fo=operating frequency Rs=effective series resistance of tunnel The adjustable loop of all the applicators herein shown is preferably of dimensions and disposition insuring that, throughout its range of adjustment, as shown by the fullline portions of the curves (Fig. 9), the mutual inductance between the anode circuit and the applicator is supraoptimum. The loop in such cases, nevertheless, has an area of lesser size than the area subtended by the loop between the inductance element and the opposing wall structure of the housing, where the area subtended by the loop is the space between the inductance element formed by the fin structure and the opposing wall structure with the upper and lower limits corresponding with those of the loop.

With the tunnel construction, supraoptimum coupling is readily obtainable since all of the high-frequency magnetic flux encircling the high-frequency current in the fin must pass through the space provided between the fin and the walls of the tunnel, and since the loop may be dimensioned and disposed to intercept a large percentage of the total flux. This close coupling is obtainable even with a single-turn loop, which may be a wide strap of low inductance, facilitating satisfaction of the requirement that the anode circuit frequency for many dielectric heating applications should be substantially higher than the resonant frequency of the applicator. In all cases, the anode circuit frequency should be non-harmonically related to the resonator frequency.

With supraoptimum coupling, for any given adjustment of the anode circuit loop throughout most of the operating range, the electrode voltage does not substantially vary with change in the loaded Q of the tunnel applicator as would occur, for example, upon change of the electrical characteristics of a dielectric load during its heating or, in a conveyor-fed tunnel, upon change in the number of load objects moving between the heating electrodes of the tunnel applicator. For example, referring to Fig. 9, it may be assumed that the work to be heated normally causes the tunnel applicator to have an apparent powerfactor of 0.002 or 0.2% and that an electrode-potential of 15,000 volts is required to heat that load at the desired rate. Accordingly, the mutual inductance is set at the corresponding value X (Fig. 9) prior to or during the early stage of heating. Should for any reason the apparent power-factor of the tunnel drop to 0.05%, the heatingelectrode potential rises only to about 17,200 volts, as indicated by Rise on the right-hand side of Fig. 9.

This is in marked contrast to the high rise in heatingelectrode voltage occurring if, in accord with prior practice, the coupling is adjustable in a range between zero and optimum. In such latter case, the mutual inductance would be initially adjusted :to the value Y, below optimum coupling (Fig. 9), to obtain the required 15,000 volts on the heating electrode. Now, upon reduction of the apparent power-factor of the applicator to 0.05%, the electrode voltage rises to over 29,000 volts (as indicated by Rise on the left-hand side of Fig. 9), an increase in electrode voltage of more than With the relatively '9 low Qs attainable with conventional load circuits, the voltage rise incident to decreased power-factor of the load, though not this great, is large and is added to by a substantial voltage rise due to decrease in dielectric constant of the load capacity which accompanies removal of load.

With the oscillator circuit 248, as also with circuits 24A and 240, when using supraoptimurn coupling the voltage change incident to change in dielectric constant is opposite in sense to that due to change in power-factor and consequently the actual rise, due to both effects, is less than the X Rise of Fig. 9. By way of-explanation, the decrease in capacitance incident to removal of load causes a decrease in tunnel electrode voltage (Fig.

Both the number and size of the work objects and the power-factor of the work material as disposed between the heating electrodes determine the apparent powerfactor of the loaded tunnel for any given electrode spacing. Substantial constancy of the electrode voltage for a selected coupling adjustment of mutual inductance is of great advantage when, as indicated in Fig. 2, the objects 193 to be heated are carried through the tunnel applicator on a conveyor belt 63B, or equivalent, becausethe apparent power=factor of the tunnel may vary from a very low value corresponding with the power-factor of the lightly loaded tunnel, which maybe as low as 0.02%, to a substantially higher value corresponding withthe powerfactor of the heavily loaded tunnel, which may be 1.0%, a range of variation of 50 to 1. Otherwise stated, at times the belt 638 may be practically coveredwith load objects, such as sand cores or plastic prefortns, whereas at other times there may be only a few objects, or non, between the heating electrodes.- With supraoptimum coupling, there is substantially uniform heating despite large vari ations in load density.

When the work is of high power-factor, such as above 2% and of area comparable to the area of the large heating electrode, the electrode spacing should be adjusted to decrease the apparent power-factor of the applicator.

Otherwise, the power-factor of the heavily loaded tunnel might, in extreme cases, be so high that the anode current could not be reduced, by adjustment of the anode loop, to safe'value.

To obtain the operational advantages of supraoptimum coupling, the loop of any of the various applicators shown should be so constructed that its minimum effective area provides supraoptimum coupling. This determines the maximum electrode voltage, for a given electrode spacing, and the corresponding unloaded Q, obtainable throughout the range of adjustment of the effective loop area as effected by adjustment of a shorting bar, by swinging of the loop in whole or in part, or by any other way. From any intermediate point in the range of ad justment, increasing the effective loop area will reduce the electrode voltage and decreasing the loop area will increase the electrode voltage.

What is claimed is:

1. In a high-frequency dielectric heating system, the combination of a power-receiving circuit comprising a resonant applicator having conductive wall structure forming an electrically conductive housing and having therein capacitance and inductance structures respectively including spaced electrodes of extended area for the heating of dielectric Work disposed in the electric field between the electrodes and an inductance element projecting into the interior of said housing in spaced relation with said wall structure to provide for the magnetic field encircling said inductance element an unobstructed path around and lengthwise thereof, one of said electrodes being formed by or electrically connected to adjacent wall structure of said housing, a second of said electrodes being disposed at and electrically connected to the inwardly projecting end of said inductance element in spaced relation to all wall structure of said housing and being electrically connected with said wall structure through 'said inductance element, and means including at least portions of said wall structure electrically interconnecting said capacitance and inductance structures to complete said power-receiving circuit and atfording a high unloaded Q thereof; high-frequency powersupply means having a power-delivery circuit, and means providing a power-transfer coupling loop included in said power-delivery circuit and disposed within the unobstructed path of said magnetic field encircling said inductance element to provide a loop area of lesser size than the area subtended by the loop between said inductance element and the opposing wall structure of said housing, said loop-providing means including a stationary part and also including a part adjustable relative to said stationary part to change the area of said loop so to control the high-frequency potential difiierence of. said heating electrodes.

2. The system of claim 1 in which said high-frequency power-supply means includes a high-power oscillator tube requiring anode cooling liquid, and in which said coupling loop is formed by a strap conductor having piping for the cooling liquid attached along the edges of said strap conductor.

3. The system of claim 1 in which said high-frequency power-supply means includes a high-power oscillator tube having an output circuit comprising said powerdelivery circuit, said tube requiring anode cooling liquid, said power-transfer coupling loop being connected in the anode circuit of said tube with said stationary part of said loop comprising a fixed section of strap conductor and piping for said cooling liquid attached to said strap conductor and said adjustable part of said loop being devoid of piping for cooling liquid.

4. The system of claim 3 in which said piping for said cooling fluid is attached along the edges of said strap conductor.

5. The system of claim 1 in which said stationary part itself forms a continuous loop and said movable part varies the effective area of said loop formed by said stationary part.

6. The system of claim 5 in which said movable part is generally U-sh'aped and which is swingable from a position closely adjacent to one side of said loop to positions angularly disposed with respect thereto and in flux-varying relation with respect to the etfective area of said loop.

7. The system of claim 1, wherein said movable part of the loop-providing means is swingable relative to said stationary part of the loop-providing means.

8. The system of claim 1, wherein said movable part of the loop-providing means comprises a member slidable on said stationary part of the loop-providing means.

9. The system of claim 1, wherein said movable part of the loop-providing means is connected in shunt with said stationary part.

10. The system of claim 1, wherein said movable part of the loop-providing means is connected in series with said stationary part.

11. A high-frequency heating system comprising a high-power oscillator tube for supplying high-frequency power to work between spaced heating electrodes, said tube requiring anode cooling liquid, a coupling loop in the anode circuit of said tube comprising a fixed section of strap conductor and piping for said cooling liquid attached to said strap conductor, and a resonant applicator comprising spaced electrodes and conductive structure electrically interconnecting said electrodes, said loop being disposed in position to be traversed by the highfrequency magnetic field encircling said structure and having a movable part devoid of piping for cooling liquid, which movable part is adjustable to vary the area of the loop so to control the high-frequency potential-difference of said heating electrodes.

12. A high-frequency heating system as in claim 11, which said movable part is a loop section pivotally connected at' its opposite ends to said strap conductor of the fixed section of the loop.

13. A high-frequency heating system as in claim 11 in which said movable part is a loop section pivotally connected at its opposite ends respectively to part of said applicator structure and to said strap conductor of the fixed section of the loop.

14. A high-frequency heating system as in claim 11 in which the movable structure is a shorting bar adjustably connected at its opposite ends to said strap conductor of the fixed section of the loop.

15. An applicator for high-frequency electric heating of dielectric materials comprising spaced electrodes for receiving material therebetween, at least one of which electrodes is movable, conductive structure electrically interconnecting said electrodes and having substantial inductance cooperative with capacity-means including said electrodes to form a resonant circuit, and a power-transfer coupling loop disposed in position to be traversed by the high-frequency magnetic field around a portion of said structure, said loop being comprised of a stationary part and also a part movable relative thereto to vary the area of the loop, said movable part being swingable about an axis normal to the direction of movement of said one electrode.

16. An applicator as defined in claim 15, wherein said movable part is generally U-shaped and is swingable from a position wherein it is between said stationary part and said one electrode, affording maximum loop area, to an opposite position affording minimum loop area.

17. An applicator for highfrequency electric heating of dielectric materials comprising a reentrant cavity reso' nator having an electrically conductive housing with reentrant structure projecting into the interior thereof, inductance structure within the housing and comprising at least one inductive element projecting into the interior of the housing and constituting at least part of said reentrant structure, spaced electrode structures cooperative to provide electric field space within the housing, one of which electrode structures is disposed at the inwardly projecting end of said inductive element in spaced relation to the wall structure of the housing and electrically connected with the wall structure of the housing through said inductive element, said wall structure providing a low resistance, low reactance path completing a resonant circuit which includes said inductance structure and said electrode structures and the frequency of which is predominantly determined by the inductance of said inductance structure and the capacitance between said electrodestructures, said housing serving as a shield to confine the electric field produced between said electrode structures and the magnetic field encircling said inductive element, and structure providing a coupling loop disposed in said housing so as to be traversed by a magnetic field encircling said inductive element, said loop-providing structure including a stationary part and a movable part, which latter is movable to vary the efiective area of said loop so as thereby to vary the amount of magnetic field flux traversing the loop.

18. An applicator for high-frequency electric heating of dielectric materials comprising an electrically conductive housing, inductance structure therein comprising at least one fin inductor projecting into the interior of said housing, spaced electrode structures cooperative to provide electric field space within the housing, one of which electrode structures is disposed at the inwardly projecting end of said fin inductor in spaced relation to walls of the housing and electrically connected with the wall structure of the housing through said fin inductor, said wall structure providing a low resistance, low reactance path completing a resonant circuit which includes said inductance structure and said electrode structures and the frequency of which is predominantly determined by the inductance of said inductance structure and the capacitance between said electrode structures, said housing serving as a shield to confine the electric field produced between said electrode structures and the magnetic field encircling said inductance structure, and means providing a single turn coupling loop within said housing in position to be traversed by said magnetic field, which loop includes a relatively stationary loop section and also includes an element movable relative thereto to vary the area of the loop.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,595 Zottu Mar. 30, 1948 2,504,956 Atwood Apr. 25, 1950 2,504,969 Ellsworth Apr. 25, 1950 2,506,626 Zottu May 9, 1950 2,684,433 Wilson July 20, 1954 

